Like Ard, I'm also not an expert at diagnosing casting errors on the internet, but two very common errors for beginners is beginning the forward cast before the back cast has completed (i.e. the crack sound). One way to help solve that is to watch the back cast "complete" before you start the forward cast. Another common error is to not come to a stop on both the forward cast and then again on the back cast, make sure you are coming to a stop on both the forward and rear casts.
You might benefit from watching this Orvis video on casting mistakes.Correcting Common Casting Mistakes